1. Field of the Invention
The present invention relates to diode power sensors. More particularly, the present invention relates to diode power sensors designed to measure power over a wide dynamic range.
2. Description of the Related Art
Diode power sensors take advantage of a square law operating region of a diode to measure power. The current (I) vs. voltage (V) equation for a diode is typically expressed in an exponentional form according to the equation
I=Io(e(xcex7V/kT)xe2x88x921)xe2x80x83xe2x80x83(1) 
where Io and xcex7 are constants whose values depend on the details of the diode, T is the diode""s temperature in Kelvins and k is Boltsmann""s constant. FIG. 1 shows an ideal I-V curve for a Schottky or PN-Junction type diode following the parameters of equation (1).
A diode""s I-V behavior can also be represented in terms of a polynomial series,                     I        =                              ∑                          n              =              1                        ∞                    ⁢                      xe2x80x83                    ⁢                                    a              n                        ⁢                          V              n                                                          (        2        )            
where the an values are chosen to suit a particular diode being considered. For a diode receiving a sinusoidal voltage, average current can be calculated using the second order term of equation (2), while other terms of equation (2) can be neglected over a portion of the diode I-V curve. Equation (2) can, thus, be simplified for average sinusoidal current as:
Iavg=a(V2)avgxe2x80x83xe2x80x83(3) 
The portion of the I-V curve where a diode operates according to equation (3) is referred to as the square law region.
A power sensor which measures RMS power can be constructed using a Schottky or PN-Junction diode to take advantage of the I-V square law relation of the diode. RMS power can then be determined by measuring average diode current. Power is determined according to the equation
P=(V2)avg/2Rxe2x80x83xe2x80x83(4) 
where P is average power, and R is the load resistance of typically 50 xcexa9. Measured average current Iavg from a diode is related to the average of the square of the diode voltage V according to equation (3), and the average of the square of V is related to average power using equation (4). Therefore average power P can be determined from average diode current Iavg according to the equation
P=Iavg/2aRxe2x80x83xe2x80x83(5) 
The power range over which a real diode operates according to the square law is limited. The typical square law operating range for a real diode is approximately xe2x88x9270 dBm to xe2x88x9220 dBm. An alternative method for measuring true RMS power is to use more expensive peak sensor/meter systems. The signal channel for these peak sensor/meter systems faithfully follows the voltage envelope for a modulated signal. Measured voltage values along the envelope are averaged to get true RMS average power. The sensor/meter systems are limited because measurements from a signal that is modulated at a higher frequency than a sensor/meter system bandwidth will produce an inaccurate measurement.
In accordance with the present invention, a diode power meter which measures RMS power using the square-law relation for a diode is provided which can measure power over a much greater range than the square-law dynamic range for a single diode.
A power meter in accordance with the present invention includes multiple diodes to enable measurement of RMS power over an 84 dB or greater range. The power meter also includes a manifold made up of power dividers to distribute an input signal to the diodes.
In one embodiment, power dividers are included in the power distribution manifold along with attenuators. In one particular version two power dividers are included with a first power divider distributing power to a first one of the diodes, and a second power divider distributing power to the second and third diodes. The first power divider is connected without attenuation to the first diode. The second power divider is connected to the second diode through an 11 dB attenuator, and to the third diode through a 28 dB attenuator. Including the 6 dB attenuation from the power dividers, the total attenuation to the first diode is 4 dB, the total attenuation to the second diode is 23 dB, and the total attenuation to the third diode is 40 dB.
With a power meter providing such attenuation, the first diode can operate in its square law range for measurements of signals with power from xe2x88x9264 dBm to xe2x88x9214 dBm, the second diode can operate in its square law range for signals with power from xe2x88x9214 dBm to +3 dBm, and the third diode can operate within its square law range for signals with power from +3 dBm to +20 dBm. By measuring average current from the appropriate diode, a power sensor will then have a +20 dBm to xe2x88x9264 dBm operating range for a total of 84 dB.
The square law power range for the second and third diodes are overlapped more to improve measurement speed for the embodiment described. A power meter requires more averaging when measuring a low detected voltage because of noise. Limiting use of lower detected voltages from the second and third diodes allows a system to be faster in a power range such as +20 dBm to xe2x88x9214 dBm because less averaging is needed. The trade-off is less dynamic range covered by the same number of diodes.
In an alternative embodiment, unequal power dividers are used in the distribution manifold to provide greater operating range sensitivity and to eliminate the need for separate attenuators. In one particular version two power dividers are included to distribute power to three diode detectors. A first power divider distributes power to a first one of the diodes and the second power divider, and the second power divider distributes power to the second and third diodes. The first power divider provides a xe2x88x924 dBm attenuation to the first diode, and a xe2x88x9217.86 dBm attenuation to the second unequal divider. The second power divider then provides a xe2x88x923.14 dBm attenuation to the second diode, and a xe2x88x9220.14 dBm attenuation to the third diode. With such attenuation, a power sensor with such unequal diodes operates from +18 dBm to xe2x88x9266 dBm, or over a total of 84 dB.
By taking advantage of the square law operating range of diodes, the power meter of the present invention can measure RMS power with lower cost components than a peak sensor/meter system because of the high cost associated in building a wide video bandwidth system.